After conducting some research, I was unable to find any official information regarding a boat ramp named CR 21B 3496 in Bradford County, Florida. It is possible that the name is outdated or that it is a local name that is not recognized on a broader scale. Therefore, I cannot provide accurate information regarding the kind of boat ramp it might be, its width, the body of water it services, or the types of watercraft that are permitted to use it.

In cases like this, it is always best to consult with local authorities or boating associations to obtain accurate information.